Device for Production of a Polychromatic Light over a Continuous Spectrum by Excitation at One Wavelength

ABSTRACT

A device for producing a polychromatic light including an optical pump that delivers a first radiation at a first wavelength; a light guide; and a selective injector that injects the radiation into the guide, the guide being arranged to generate a harmonic corresponding to the selected injector and provide a polychromatic light at an outlet, by non-linear excitation of the first radiation and the harmonic.

A device for the production of a polychromatic light over a continuous spectrum by excitation at one wavelength

The present invention relates to a polychromatic light producing device.

The present invention more particularly concerns the production of a wide spectrum white light.

In this field, the utilization of a source directly emitting a wide spectrum white light such as an incandescent lamp or a quartz lamp is already known in the prior art. The direct utilization of such sources has numerous disadvantages such as, for example, the overall dimension, the isotropic emission or the weak luminance.

In order to obtain a wide-band white light, it has been attempted to associate in parallel a great number of mono-frequency laser diodes, but it should be understood that in this case, the spectrum is not continuous. Besides, variations in intensity may occur between the diodes.

Another known mean for obtaining such a light consists in pumping a micro-structured optical fiber with a mono-frequency laser which works in pulsed operation, with pulses of the order of a nanosecond, a picosecond or a femtosecond for distribution frequencies between the kilohertz to the megahertz. It is known that such a pumping induces non-linear effects in the micro-structured fiber which allow the generation of photons having various wavelengths. This type of source has been described in the publication by J. K Ranka et al., Optics Letters, Vol. 25, No. 1, p. 25-27, 2000 and in the document by J. C. Knight et. al., Optics Letters, Vol. 26, No. 17, pages 1356-1358, 2001. The disadvantages of such a source are the overall dimensions and the cost which prevents its utilization in an integrated product.

Besides, the inventors proposed a first solution for obtaining a wide spectrum white light. This solution is disclosed in the French application no 04 02589, filed on Mar. 12, 2004 and not yet published. In this document, the inventors disclose, as illustrated in FIG. 1, a device comprising an optical pumping module MP in charge of providing a radiation having at least two different excitation wavelengths λI and λ2. It also comprises light guidance means GL which, when they are excited by a radiation including several wavelengths provided by the pumping module MP in a non-linear light-material interaction regime, provide a polychromatic light corresponding to a continuum at their outlet SGL.

As a matter of fact, and as explained in the French application no 04 02589, in a mono-modal fiber using a negative dispersion regime, the pumping by a mono-frequency source gives rise to the generation of Raman discrete lines for high wavelengths. On the contrary, in the case of a pumping with at least two judiciously selected wavelengths, the inventors demonstrated that a symmetrical homogeneous spectral spreading appeared on either side of the one the pumping wavelengths, as well as a minimization of the Raman components. This modification is mainly the result of a competition phenomenon between the non-linear effects. The spectral spreading can be explained by the combination of the phase self-modulation, the crossed phase modulation and the parametric effects (four wave mixture). The addition of a second pumping power then makes it possible to initiate initial phase matching between the wavelengths centered around λI and λ2 and the occurrence of the parametric effects which have an occurrence threshold lower than that of the Raman effect. As illustrated in FIG. 1, such a device thus comprises pumping means MP which generate two wavelengths λI and λ2 at the outlet SMP. Such radiation then goes into the guidance means GL through the inlet EGL and a wide-spectrum white light is emitted at the outlet of the guide GL through the outlet SGL.

Although the invention. described in the application no 04 02589 already solves many problems in the field of obtaining a white light, it requires a double pumping which requires either a double pump or a frequency doubler which can be a problem.

The present invention is thus intended at further simplifying the previously described device while still having satisfactory results as regards the emitted light and at making it possible to obtain a spectrum without any discontinuity.

For this purpose, the present invention is of the type described hereabove, and it is remarkable in the broadest sense, in that it relates to a device for generating a polychromatic light comprising optical pumping means and light guidance means, the optical pumping means being capable of producing a first radiation at a first wavelength, characterized in that it includes a selective injection means for injecting said radiation into the guidance means, said guidance mean being so arranged as to generate a harmonic corresponding to said selective injection and to produce at an outlet a polychromatic light by a non-linear excitation of said first radiation and said harmonic.

Preferably, the guidance means include a light guide for example of the optical fiber type.

Advantageously, in order to generate said harmonic while transmitting the first radiation, the guidance means or more precisely the fiber, are substantially mono-modal at the level of said first wavelength and substantially multimode at the level of the wavelength corresponding to said second harmonic.

Preferably, in order to raise the generation rate of said harmonic, the guidance means, or more precisely the fiber, are doped with germanium and/or submitted to a poling treatment.

Advantageously, in order to control the propagation of the radiations and their spectral properties, the guidance means include a micro-structured fiber.

The invention will be better understood when reading the description given hereinunder only for the explanation of one embodiment of the invention and while referring to the appended Figures in which:

FIG. 1 illustrates a device such as described in the prior French application no 04 02589 not published yet;

FIG. 2 illustrates a device according to the present invention;

FIG. 3 shows a cross-sectional view of a fiber used according to the invention;

FIG. 4 illustrate an exemplary super-continuum embodied while using the device according to the invention.

While referring to FIG. 2, the invention comprises a device D dedicated to the generation of a polychromatic wave.

Such device D firstly includes an optical pumping module MP in charge of producing a radiation. Quite originally, it is sufficient, according to the invention, for such pumping means to produce a radiation having only one wavelength λI. It should be understood that these means can produce more than one wavelength, but the technical problem of the emission of a white light is solved according to the invention with only one wavelength.

For example, these pumping means can be in the form of a monochromatic laser source SL. It should be understood that any pumping means capable of producing a radiation having at least one wavelength can be used.

As an example and in order to best describe the invention, the emission of a radiation at 1064 nm at the outlet SMP of the pumping means MP has been considered. Besides, the pumping module used can be operated either in pulsed mode or in a continuous or quasi-continuous mode.

The device D according to the invention also includes light guidance means GL in charge of producing, at their outlet SGL, a polychromatic light corresponding to a continuum.

Here, a polychromatic light means a light the spectrum of which is substantially continuous and not discretized as regards its wavelength on a wide band.

Preferably, the guidance means GL used is an optical fiber and more generally a light guide the non-linear properties of which depend on its structure and its dimensions. For example, a so-called micro-structured fiber is used. Such type of optical fiber has an opto-geometrical structure which can be modified upon its creation and which makes it possible to control the dispersion profile and allows to guide the light on the whole transmission band of the fiber material.

For example, the optical fiber GL can be a hole type optical fiber, similar to that illustrated in FIG. 3 in a cross-sectional view. Such type of optical fiber is more particularly described in details in patent EP 1 148 360. Such micro-structured fiber is composed of hollow channels CC parallel to the longitudinal axis of the optical fiber GL and positioned adjacent to each other around the fiber core CF, according to a selected pitch, so as to define a meshed structure (which can also be called a photonic crystal), and made in the material composing the sheath GF which surrounds the core CF of the optical fiber GL. It should be reminded that a photonic crystal can be a forbidden energy gap structure (when guiding is provided by the transversal meshing), composed of diffracting elements (here, the hollow channels DC) defining a mesh and the physical properties of which make it possible to control the light propagation. Such fibers can, for example, be obtained by extrusion in a die, as described in the work by Jean-Michel Lourtioz, “Les cristaux photoniques”, p. 324, Editions Hermes, 2003.

According to another embodiment, a non micro-structured fiber can be used. Nevertheless, in order to obtain wavelengths in the visible region, it must have a dispersion zero shifted towards the lower wavelengths.

In order to obtain the requested wavelengths and more particularly the infra-red and the visible spectra, the selected fiber is either totally mono-modal, or mono-modal for the pump wavelength and multimodal, and more particularly bi-modal for the region corresponding to the high order harmonics, for example in the visible spectrum.

Those skilled in the art understand how important the selection of the fiber is, as regards its excitation modes. As a matter of fact for a mono-modal fiber, the inter-mode phase matching are impossible and push up the occurrence threshold of the non-linear effects (a second harmonic generation rate which is greater than 3%-4% is then required). Using a bi-modal fiber in the visible region and a mono-modal fiber in the excitation region (infra-red) makes it possible to lower the occurrence threshold of the non-linear effects (parametric effects) thanks to the evolution of the dispersion curve between the various modes.

On the contrary, in a very multi-modal fiber, the distribution of the incident power on several modes reduces the efficiency of the power transfer towards the low wavelengths. And it should be understood that it is impossible to excite a single mode in the visible region if the fiber is very multi-modal for the pump wavelength.

According to the invention, the signal delivered by the exciting source MP is injected into the light guidance means GL, for example in the form of a micro-structured optical fiber. A particular excitation called a “selective injection” makes it possible to obtain the excitation of a transverse mode of a single upper order (for example the so-called mode “LP11”) other than the fundamental (so-called “LP01”) mode in the visible region.

Such selective injection is for example embodied using converging lens L as illustrated in FIG. 2. The transversal motion of the lens and a special orientation of the beam injected into the fiber thus makes it possible to excite a fundamental transverse mode (TEM00) or a single high order (LP11 or LP21).

Besides, the micro-structured fiber is embodied so as to have a high second harmonic generation rate, for example thanks to the air-silica interfaces and to the impurities trapped in the silica. A process of optical, electrical or thermal “poling” (English word meaning a poling in the field of metal processing) of such fiber makes it possible to substantially increase the second harmonic generation rate. This non-linear effect of frequency doubling is directly used to create the super-continuum.

More particularly, the first experiment of the second harmonic generation as observed by Sasaki Y. 10 and al. (Appl. Phys. Lett., 39 no 6 (1981) pp 466-468) in a silica fiber of the core of which is doped with germanium. The polarization is induced by the illumination by a Nd:YAG laser (1,064 micron; peak power 0.8 KW) for several hours. The conversion ratio is of the order of 1%.

A few years later, the experiment was extended by Osterberg and Margulis (Opt. Lett., 11 no. 8 (1986) pp 516-51) on the germanium-doped silica fibers irradiated by the Nd:YAG laser (1,064 micron; peak power 0.5-1 KW; =50 ps). The conversion ratio is much higher and of the order of 5%.

In short, models have demonstrated that this spontaneous second harmonic generation in the fibers is based on the defects and impurities leading to local birefringences. The second harmonic generation is then created in a strong phase mismatching regime and in an incoherent manner from one place to the other.

Preferably, and according to the invention, such spontaneous effect is increased by an optical, thermal or electric poling effect making it possible to induce a strong local field in the fiber. Such means make it possible to further improve the results thanks to a better generation of the second harmonic.

According to another embodiment which can be combined with the precedent poling, hole type fibers can be used in which the interfaces between silica and the sheath can be a spontaneous source for the generation of the second harmonic.

Those skilled in the art will understand that the performances of the present invention are increased with the second harmonic generation rate. The higher this rate the brighter the spectrum in the visible region for a given phase matching. High generation rates for example between 2% and 10% may for example be obtained by a doping of the fiber with germanium and/or by a poling.

As illustrate in FIG. 4, those skilled in the art will note that the super-continuum obtained when using the device according to the invention has all the frequential components of the visible field as well as in the close infra-red and this with a very uniform power.

Using the device according to the invention this continuous wide spectrum can be generated on wavelengths between 300 nm and 1.8 μm for substantially uniform power levels.

The laser source is for example of the YAG type and can be of mono-modal or multimodal, nanosecond, picosecond, microsecond or continuous type. The fiber used to embody the spectrum of FIG. 4 measures approximately 4 meters.

Those skilled in the art will understand that the types of application of the present invention are very numerous. As an example, the field of telecommunication, biophotonics and more particularly for diagnostic or cellular sorting, ophthalmology or consistent optical tomography or confocal microscopy can be mentioned.

The invention has been described hereabove as an example. It should be noted that those skilled in the art can easily make various modifications in the invention without leaving the scope of the patent. 

1-4. (canceled)
 5. A device for producing a polychromatic light comprising: an optical pump that delivers a first radiation at a first wavelength; a light guide; and a selective injector that injects the radiation into the guide, the guide being arranged to generate a harmonic corresponding to the selected injector and provide a polychromatic light at an outlet, by nonlinear excitation of the first radiation and the harmonic.
 6. The device according to claim 5, wherein the guidance means include an optical fiber.
 7. The device according to claim 6, wherein the optical fiber is a micro-structured fiber.
 8. The device according to claim 5, wherein the guidance means are substantially mono-modal for the first wavelength and substantially multimode for the harmonic to provide a polychromatic light at one outlet. 